1. Field of the Invention
The present invention is directed to a water-based paint composition, and more particularly to a water-based paint composition which is suitable for re-use after being recovered from a flushing water utilized in a coating booth.
2. Description of the Prior Art
In the field of spray-coating an article within a coating booth, a large amount of coating or paint material remain undeposited away from the article and causes a dust of paint material within the booth. Such paint material is collected by the use of flushing water as being dispersed or dissolved therein. As the paint material is recovered in a large amount, disposal of the paint material with the flushing water cause an economical loss of material and even environmental pollution. To avoid this problem, it has been proposed to recover the paint material from the flushing water for re-use as disclosed, for example, in Japanese patent non-examined early publication (KOKAI) NO. 49-51324. The publication discloses a process of recovering a water-based paint composition comprising a water-soluble resin such as alkyd resin and acrylic resin and a pigment from the flushing water by the use of a reverse osmosis or ultrafiltration membrane, in which the paint composition diluted with the flushing water is filtrated through the membrane in order to increase the concentration of the water-soluble resin and the pigment up to a level approximate to an original concentration for recycling of the paint composition.
However, this filtration process fails to successfully recover a water-soluble organic solvent which is normally contained in the water-based paint composition in several percent for dispersing the water-soluble resin in water, because of that the organic solvent will largely pass through the membrane together with the water. With this result, the recovered paint composition suffers from less concentration of the organic solvent which may cause separation or aggregation of the water-soluble resin or vehicle and the pigment during condensation, leaving unacceptable imbibition in a coating with thus recovered paint composition.
In order to eliminate this problem, the water-based paint composition is proposed to have increased water solubility by the incorporation of a water-soluble resin such as alkyd and acrylic resin having high acid and hydroxyl values. However, this scheme is found to pose another drawback of lowering resistance to moisture and hot water, and also lowering shock resistance.
Further, it is noted that because of that the water-based paint composition is unable to adjust drying speed as opposed to solvent-thinned paints, sagging is likely to occur during the application of the paint, particularly in a high humid environment. When sagging is prevailing, it is hard to apply a relatively thick coat by a single coating operation, thereby lowering coating workability. In order to eliminate the sagging of the paint, it has been attempted to incorporate a thickening agent in emulsion form. However, the thickening agent tends to coalesce with time, which may sometimes produce a grainy surface or the like poor coating property.